Semiconductor device having trenches filled with a semiconductor having an impurity concentration gradient

Abstract

A single crystal semiconductor layer of a first conduction type is disposed on a surface of a semiconductor substrate. A plurality of trenches are provided in the semiconductor layer to form a plurality of first semiconductor regions of the first conduction type at intervals in a direction parallel to the surface. An epitaxial layer is buried in the plurality of trenches to form a plurality of second semiconductor regions of a second conduction type. The plurality of second semiconductor regions each includes an outer portion with a high impurity concentration formed against an inner wall of the trench, and an inner portion with a low impurity concentration formed inner than the outer portion.

1. A semiconductor device, comprising:
 a semiconductor substrate; 
 a first semiconductor region, of a first conduction type formed on a surface of the semiconductor substrate; 
 a plurality of trenches formed in the first semiconductor region at a certain interval; and 
 a second semiconductor region of a second conduction type buried in the plurality of trenches, the second semiconductor region including an outer portion formed against an inner wall of the trench, and an inner portion formed inside the outer portion, 
 the outer portion having a higher impurity concentration of the second conduction type than the inner portion. 
 
     
     
       2. The semiconductor device according to  claim 1 , wherein the plurality of trenches each has a width that increases becoming more distant from a bottom of the each trench. 
     
     
       3. The semiconductor device according to  claim 1 , wherein the outer portion has a substantially constant thickness. 
     
     
       4. The semiconductor device according to  claim 1 , wherein the outer portion has a substantially constant thickness, and the thickness of the outer portion is less than half the width of a bottom of the trench. 
     
     
       5. The semiconductor device according to  claim 1 , wherein a bottom of the trench reaches the semiconductor substrate. 
     
     
       6. The semiconductor device according to  claim 1 , wherein a part of the first semiconductor region is formed between the bottom of the each trench and the semiconductor substrate. 
     
     
       7. The semiconductor device according to  claim 1 , further comprising:
 a base layer of the second conduction type formed on the first semiconductor region; 
 a source layer of the first conduction type formed on the base layer; and 
 a gate electrode formed on the source layer and the first semiconductor region via a gate insulation film.